Research on Impacts of Rainfall on Free Flow and Non-Free Flow of Expressway

Rainfall, as the most common adverse weather, has many bad effects on expressway traffic flow. At present, the research on the impacts of rainfall on expressway traffic flow mainly uses free flow speed, traffic capacity to characterize the degree of impacts. For free flow and non-free flow, whether the impact of rainfall is consistent is not clear. In order to explore the impact of rainfall on free flow and non-free flow of expressway, the traffic flow data under normal weather, light rain and moderate rain conditions were collected in Xi’an Ring Expressway. The traffic flow was divided into free flow, steady-state flow and congested flow by using ν/C (volume to capacity) ratio as an index. The vehicle speed and headway were selected as characteristic parameters. SPSS (Statistical Product and Service Solutions) and Origin software were used to process and analyze the data. The results show that the rainfall will reduce the vehicle speed and make the vehicle speed distribution more concentrated, and the impact is greater in the free flow state. Rainfall has little effect on the distribution of headway under free and non-free flow conditions, but for car-following vehicles, rainfall increases the mean headway.